In this paper, we consider a two-dimensional UAV-enabled industrial automation scenario in Fig. REF , where a controller needs to send command messages to a distant robot that conducts an experiment in a multi-hazard area. For the safety of workers, shelters such as thick cement/metal walls are built between the robot and the controller. Hence, the channel gain between the controller and the robot is weak and negligible, and requires a UAV to fly above the shelter to assist the transmission between the controller and the robot. In [1]}, we studied the problem of jointly optimizing the blocklength and location for UAV-relay communication systems, where the decoding-and-forward (DF) protocol was considered. However, additional processing time is required for the DF mode, which may not be applicable to URLLC applications. Motivated by above, we jointly optimize power and location to minimize the decoding error probability, where the relay is operating under the AF mode without the signal processing delay. The decoding error probability under short blocklength is adopted. We first prove that the decoding error probability is a monotonically decreasing function the SNR. Then, two channel models are studied: free-space channel model and the 3-D channel model. For the first one, an iterative algorithm is proposed to obtain the suboptimal solution with low complexity, and the globally optimal solution is obtained in closed form when the SNR is extremely high. Simulation results show the performance advantages of our proposed algorithms.
